Upgrade Django to 3.2

[rtibbles]

Summary

• Upgrade Django to 3.2
• Upgrade DRF
• Remove use of Django six
• Remove no longer used MorangoMPTT model custom handling
• Update fields to remove context argument
• Upgrade MPTT and create new migrations for PositiveIntegerField etc.
• Use python functools.
• Don't use schema editor as context (no longer allowed for SQLite)
• Update Django middleware name.
• Set now required output field on function.
• Update how we handle custom serialization and deserialization, following Django's own serialization framework that conditionally renders to string or leaves data types raw - allows specific fields to opt in to string serialization.
• Importantly, this handling now updates serialization and deserialization behaviour, which it did not previously.
• Update the syncsession tests to run the additional server in an isolated process, as databases were no longer being properly set in the LiveServerThread for LiveServerTestCase
• Updates the psycopg2-binary package to the latest compatible version

TODO

• Have tests been written for the new code?
• New dependencies (if any) added to requirements file

Issues addressed

Fixes #208

[rtibbles]

Oh, whoops, I left a print statement in here too!

[bjester]

LGTM and all the tests are happy 👍